1. Field of the Invention
The present invention relates generally to light emitting devices, and, more particularly, to a system and method for the monolithic integration of a light emitting device and a photodetector for low bias voltage operation.
2. Related Art
Semiconductor light emitting devices in general and vertical cavity surface emitting lasers (VCSEL's) in particular are used for many applications including electronics, communication systems, and computer systems. Lasers produce light that can be transmitted directionally. In many applications of lasers, and particularly in many VCSEL applications, there is a need to precisely control the laser output power. The output power of semiconductor lasers is primarily determined by the bias current. However, it can be significantly altered by the ambient temperature and aging of the device. For this reason, control of the output power is realized by monitoring the laser output and adjusting the laser current to maintain a specified laser output power. The light measurement is typically performed using a semiconductor photodetector, while the feedback loop is realized using an external electronic circuit. There are numerous implementations of such laser-photodetector systems, and they differ in application and performance.
The two primary design issues relating to the laser-photodetector system are the cost of the device and the ability to provide performance required for a specific application. From a cost perspective, it is desirable to build the laser and the photodetector on the same chip using the same or similar fabrication technology. This is realized by monolithic integration of the laser and the photodetector. Monolithic integration implies that the individual laser and photodetector devices are completed jointly at the wafer level. From a performance perspective, there are a number or desired qualities. The detector current should track the directional light output from the laser, while minimizing the capture of the omni-directional spontaneous emission. The relationship between the directional laser output power and the photodetector current should be stable and repeatable. For proper operation the photodetector current should be within the range needed by the external analog feedback circuit. The existence of the photodetector and its biasing should have a negligible effect on the operation of, and in particular, the modulation properties of the laser. The laser modulation and biasing should have a negligible effect on the operation of the photodetector.
Lastly, the driver circuit must be considered. In computer communications applications the minimum bias voltage is an issue of increasing importance due to the desire to reduce computer power consumption. Today's computer architectures are using 3.3 volt (V) power supplies having a lower limit of approximately 3.1V. In the future and for other applications it is foreseeable that the power dissipation will be reduced even further requiring even lower bias voltage levels.
A preferable configuration of the laser-photodetector system is one in which the laser and photodetector are independently biased from the same power supply. In order to achieve this result, the power supply voltage must be larger than the laser operating voltage, which depends on the photon energy, and the photodetector operating voltage, which depends upon the photodetector reverse bias required for efficient performance. For optical communications, the vertical cavity laser voltages range between approximately one to two volts, while the typical photodetector reverse bias voltage is between 0.5 and 1 volt. For other applications these voltages may vary.
An integrated laser and photodetector structure that enables independent biasing of the laser and the photodetector uses the lowest bias voltage. This is achievable by using a four terminal device structure in which two terminals are jointly connected to the power supply, thereby allowing arbitrary relative polarity between the laser and the photodetector. In three terminal monolithically integrated devices, the relative polarity between the laser and the photodetector is not arbitrary due to fabrication limitations.
In the past, photodetectors have been integrated with lasers with varying degrees of success. For example, some integration schemes use a photodetector and laser that have been independently fabricated on different chips. The two devices are integrated at the packaging stage, after fabrication, resulting in arbitrary relative polarity between the laser and photodetector. This integration scheme is referred to as "hybrid integration". The primary disadvantage of this approach is that the extra processing step of integrating the photodetector with the laser after fabrication undesirably adds manufacturing cost. Additionally, in many cases the relationship between the photodetector current and the laser output is neither stable nor repeatable, due to the fluctuation in the laser output beam shape.
Another scheme involves monolithic integration of a photodetector and laser where the coupling is realized using side emission, resulting in both three or four terminal devices. The main disadvantage of such devices is that the photodetector does not detect the directional laser output, but predominately captures the omni-directional spontaneous emission.
Finally, another scheme involves the monolithic integration of a laser and a photodetector where the coupling is realized by top (or bottom) emission, resulting in both three and four terminal devices.
All of the implementations result in either three terminal devices where the laser and the photodetector share a common n-side (cathode) or a common p-side (anode), which as will be shown require a relatively high bias voltage for operation and in which the laser and the photodiode are electrically coupled; or in four terminal devices, which are difficult and costly to fabricate. Therefore, a monolithically-integrated three terminal device that can operate at a low bias voltage, such as 3.3V, and enables electrical de-coupling between the laser and the photodetector is desired.
Shown in FIG. 1A is a prior art laser and photodetector combination in a three terminal configuration. Laser and photodetector 11 is essentially comprised of photodetector 12 deposited over laser 13 in a common cathode arrangement. The common cathode configuration is also referred to as PNP configuration because the semiconductor conductivity type changes twice in the structure. Laser 13 is typically a vertical cavity surface emitting laser (VCSEL). This arrangement is illustratively characterized as having two PN junctions. The first PN junction is active layer 14 located within laser 13 and the second PN junction is the absorbing layer 16 within photodetector 12. The laser 13 comprises a p-type substrate 22 on the bottom of which a p-type contact layer 21 is deposited. Over the substrate 22 is p-type mirror 23. Active region 14, which includes an n-type material and a p-type material separated by a light generating medium is grown over p-type mirror 23. Over active region 14 is n-type mirror 24, over which is grown n-type contact material 26.
Immediately upon n-type mirror 24 is n-type layer 31 of photodetector 12, over which absorbing layer 16 and p-type layer 32 are grown. Layers 32, 16, and 31 comprise a photodetector having a PIN structure. Over the p-type material 32 is p-type contact material 33. A fraction of the light emitted from the laser 13 is absorbed in the photodetector 12 and the balance is emitted from the device in the direction of the arrow shown in FIG. 1A.
FIG. 1B is a schematic representation of the prior art laser and photodetector of FIG. 1A including exemplary external circuitry associated therewith. The dotted box surrounding diode 36 and diode 37 illustrates laser and photodetector 11, where diode 36 represents the laser 13 and the diode 37 represents the photodetector 12. Laser driver transistor 41 and the current mirror formed by transistors 38 and 39 are illustratively depicted using NPN bipolar transistors. The use of NPN transistors in the amplifiers is common due to the requirement for high speed combined with the higher gain of the NPN transistor with respect to a PNP configuration (although the use of PNP transistors is not excluded).
The minimum bias voltage V.sub.CC necessary for the operation of this circuit is greater than the sum of the smallest individual PN junction voltages and can be expressed by the following relationship. EQU V.sub.CC &gt;V.sub.L +V.sub.PD +V.sub.CM (Equation 1)
where, V.sub.L is the VCSEL forward bias voltage across laser diode 36 at the laser operating current, V.sub.PD is the reverse bias across photodetector diode 37, and V.sub.CM is the forward bias on the transistor 38 in the current mirror 42 formed by transistors 38 and 39. Typically, the values of these voltages are V.sub.L.apprxeq.1.7V, V.sub.PD.apprxeq.0.5V, and V.sub.CM.apprxeq.0.8V. In this example, V.sub.CM.apprxeq.0.8V corresponds to a silicon bipolar transistor and may be greater or less than 0.8V depending upon the type of transistor and the material of which the transistor is constructed. This yields a minimum bias voltage V.sub.CC &gt;3V. It is desirable to minimize the bias voltage V.sub.CC such that overall system voltage consumption is minimized. The aforementioned laser and photodetector leaves very little cushion in a system having a 3.1V minimum power supply.
Furthermore, in the foregoing example, transistor 41 drives both laser 36 and photodetector diode 37, in which the parasitic capacitance of the photodetector 37 loads transistor 41, thus limiting the frequency response of the laser. In addition, the adjustment of the operating point of the laser 36 to reach a specified output power inherently modifies the operating point of the photodetector 37, which is a highly undesirable side-effect of this configuration. Thus, an unaddressed need exists in the industry for a three terminal monolithically integrated laser and photodetector arrangement that operates using a low bias voltage and that provides electrical de-coupling between the laser and the photodetector.